Adaptive echo cancelers operate on an incoming signal to generate an impulse response characteristic in accordance with a prescribed algorithm. Existing adaptive echo cancelers are able to model an impulse response of a limited interval, for example, 16 milliseconds. In some applications it is desirable to model an impulse response having an interval greater than the available interval of an individual adaptive echo canceler. One example is in applications where the round trip electrical delay encountered in a telephone transmission channel is greater than the impulse response interval of an individual echo canceler.
One solution to this problem is to use a "truly" cascadable adaptive echo canceler. In such an arrangement each canceler uses a common error signal to update the impulse response estimate. In order to do this the individual adaptive echo cancelers must be arranged circuit wise to facilitate inputting the common error signal to the impulse response updating circuitry. Such a truly cascadable echo canceler including an adaptive filter is manufactured by Western Electric Company. However, most presently available adaptive echo cancelers do not have this capability.
Consequently, in order to address the need for a longer impulse response interval with most existing adaptive echo cancelers a tandem arrangement is required. Attempts at tandeming echo cancelers have heretofore yielded less than desirable results. One arrangement includes having two cancelers connected in tandem with a fixed delay connected in the receive side of one of them. It was hoped that this arrangement would extend the impulse response to twice the interval of an individual echo canceler. A serious problem with this arrangement is that the echo canceler which is the first one to subtract its estimate of a reference signal from an actual reference signal has poor adaptation. This poor adaptation results because the error signal used for updating the impulse response includes the portion of the reference signal which the other echo canceler should estimate. As is known in the art, this results in unwanted misalignment noise.